POLYESTER FIBERS

Updated: April,
2004- Raghavendra R. Hegde, Atul Dahiya, M. G. Kamath

(RamaiahKotra and Xiao Gao)

1. INTRODUCTION

In 1996,
24.1 million metric tons of manmade fibers were produced worldwide. The main
volume gain took place in production of PET fibers (PET filament 9%, PET staple
4%) [1]. The primary drive for this growth is demand
for fiber and container resin. Seventy five percent of the entire PET
production is directed toward fiber manufacturing. Hoechst, Dupont and Eastman
are the three world largest polyester producers. Additional current U.S.
Polyester Fiber Producers are: Acordis Industrial Fibers, Inc.; AlliedSignal
Inc; Cookson Fibers, Inc.; KoSa; Intercontinental Polymers, Inc., Martin
Color-Fi. Nan Ya Plastics Corp., Wellman, Inc. [24] Dramatic growth in PET
fiber production is foreseen in Asia in the
near future [22].

The cost of
polyester, with the combination of its superior strength and resilience, is
lower than that of rayon. Polyester fibers are hydrophobic, which is desirable
for lightweight facing fabrics used in the disposable industry. They provide a
perceptible dry feel on the facing, even when the inner absorbent media is
saturated. As new methods of processing and bonding of PET are developed, rayon
is being replaced by polyester on the market.
According to David Harrison [2], 49% of the total nonwovens market share in the
USA
belongs to polyester staple, reaching 291 million pounds in 1996 and ranking
number one among all kind of fiber supplies. If one were to assume that the
filament fiber consumption is half of that of staple, the total PET consumption
in the USA
nonwovens industry alone would be over 450 million pounds. Even if the estimate
is not completely accurate, polyester has become the most widely used polymer
in the nonwovens industry since1995. The next most
popular was polypropylene [2]. But in 1996, poly-olefins, particularly
polypropylene (PP) moved ahead of PET fibers. They had 46% market shares in
fibers used for nonwovens, whereas, PET had a 45% share. By the end of 1998,
olefin fibers increased their share to 49% and PET dropped to 42%[25].

Mechanical
properties of nonwoven fabrics depend on many parameters, including fiber
properties, web structure and processing. It is, therefore, useful to review some
of the elementary knowledge of fiber properties and other factors like web
processing techniques and structure. What follows is a brief review of PET
fiber properties, which should serve as background information for better
understanding the subject.

2. POLYESTER FIBERS

Polyester
fiber is a " manufactured fiber in which the fiber forming substance is
any long chain synthetic polymer composed at least 85% by weight of an ester of
a dihydric alcohol (HOROH) and terephthalic acid (p-HOOC-C6H4COOH)"
[3]. The most widely used polyester fiber is made from the linear polymer poly
(ethylene terephtalate), and this polyester class is generally referred to
simply as PET. High strength, high modulus, low _shrinkage, heat set stability,
light fastness and chemical resistance account for the great versatility of
PET.

3.
POLYMER FORMATION

Polyethylene
Teraphthalate (PET) is a condensation polymer and is industrially produced by
either terephthalic acid or dimethyl terephthalate with ethylene glycol. [26]
Other polyester fibers of interest to the nonwovens field include:

(b)
Dimethyl Terephthalate (DMT), made in the early stages by esterification of
terephthalic acid. However, a different process involving two oxidation and
esterification stages now accounts for most DMT.

(c)
Ethylene Glycol (EG) initially generated as an intermediate product by
oxidation of ethylene. Further ethylene glycol is obtained by reaction of
ethylene oxide with water.

Fig.1:
Production of polyethylene terephthalate

4.
SYNTHESIS OF POLYMER

4.4 Linear Polyesters

A
representative linear
polyester, PET is polymerized by one of the following two ways: Ester Interchange:
Monomers are diethyl terephtalate and ethylene glycol. Direct Etherification:
Monomers are terephthalic acid and ethylene glycol. Both ester interchange and
direct esterification processes are combined with polycondensation steps either
batch-wise or continuously. Batch-wise systems need two-reaction vessels- one
for esterification or ester interchange, the other for polymerization.
Continuous systems need at least three vessels - one for esterification or
shear interchange, another for reducing excess glycols, the other for
polymerization.

Another way
to produce PET is solid-phase polycondensation. In the process, a melt
polycondensation is continued until the pre-polymer has an Intrinsic Viscosity
of 1.0-1.4, at which point the polymer is cast into a solid firm. The
pre-crystallization is carried out by heating (above 200oC) until
the desirable molecular weight is obtained. Later the particulate polymer is
melted for spinning. This process is not popular for textile PET fibers but is
used for some industrial fibers.

4.2 Branched and Crosslinked Polyesters

If glycerol
is allowed to react with a diacid or its anhydride each glycerol will generate
one branch point. Such molecules can grow to very high molecular weight. If internal coupling occurs (reaction of a hydroxyl group and an
acid function from branches of the same or different molecule), the polymer
will become crosslinked. Rigidly crosslinked polymers are totally unaffected by solvents.

5. FIBER FORMATION

The
sequences for production of PET fibers and yarns depend on the different ways
of polymerization (continuous, batch-wise, and solid-phase) and spinning (low
or high windup speed) processes.

5.1 Spinning Process

The degree
of polymerization of PET is controlled, depending on its end-uses. PET for
industrial fibers has a higher degree of polymerization, higher molecular
weight and higher viscosity. The normal molecular weight range lies between
15,000 and 20,000. With the normal extrusion temperature (280-290oC),
it has a low shear viscosity is 1000-3000 poise. Low molecular weight PET is
spun at 265oC, whereas ultrahigh molecular weigh PET is spun at 300oC
or above. The degree of orientation is generally proportional to the wind-up
speeds in the spinning process. Theoretically, the maximum orientation along
with increase in productivity is obtained at a wind-up speed of 10,000m/min.
Although due to a voided skin, adverse effects may appear at wind-up speeds
above 7000m/min.

5.2 Drawing Process

To produce
uniform PET, the drawing process is carried out at temperature above the glass
transition temperature (80-90oC). Since the drawing process gives
additional orientation to products, the draw ratios (3:1-6:1) vary according to
the final end-uses. For higher tenacities, the higher draw ratios are required.
In addition to orientation, crystallinity may be developed during the drawing
at the temperature range of 140-220oC [26].

5.3 Polyester Fiber Flow Chart

Fig. 2: Polyester fiber
flow chart

5.4 The latest
Polyester production (Research Method)

Dr Boncella
and Dr Wagner at The University of Florida are two scientists involved with the
study to reveal a method for manufacturing polyester from two inexpensive
gases: carbon monoxide and ethylene oxide. The polyester most commonly used
today is referred to as PET or polyethylene terephtalate. Scientists have been
successful in producing low molecular weight polyester using carbon monoxide
and ethylene oxide, but researchers still lack the catalyst - a substance that
speeds up chemical reactions - needed to make the reaction work more
efficiently. They are looking for the chemical compound that will take
molecules of low DP and create 1arger ones. Although they have had success in
the research so far, they have yet to produce commercially useable polyester
from the inexpensive gases. If this is successful, then these research findings
can be used to replace the current polyester product, getting the same
performance for a lower price. Finally, we all know that research requires
patience and a long-term effort [27].

5. STRUCTURAL
COMPOSITION OF PET

The one of
the distinguishing characteristics of PET is attributed to the benzene rings in
the polymer chain. The aromatic character leads to chain stiffness, preventing
the deformation of disordered regions, which results in weak van der Waals
interaction forces between chains. Due to this, PET is difficult to be
crystallized. Polyester fibers may be considered to be composed of crystalline,
oriented semi crystalline and noncrystalline (amorphous) regions. The aromatic,
carboxyl and aliphatic molecular groups are nearly planar in configuration and
exist in a side-by-side arrangement. Stabilization distances between atoms in
neighboring molecules are usually van der Waals contact distances, and there is
no structural evidence of any abnormally strong forces among the molecules. The
unusually high melting point of PET (compared to aliphatic polyesters) is not
the result of any unusual intermolecular forces, but is attributed to ester
linkages. The cohesion of PET chains is a result of hydrogen bonds and van der
Waals interactions, caused by dipole interaction, induction and dispersion
forces among the chains. The capacity to form useful fibers and the tendency to
crystallize depend on these forces of attraction.

The
interactive forces create inflexible tight packing among macromolecules,
showing high modulus, strength, and resistance to moisture, dyestuffs and
solvents. The limited flexibility in the macromolecule is mainly due to the
ethylene group. The extended quenched fiber does not show any early development
of crystallinity; the growth of crystals starts to occur upon drawing. A number
of basic structural models are required to represent the different states of
the fiber: amorphous (no orientation) after extrusion, amorphous (no
orientation) after cold drawing, crystalline orientation after thermal
treatment and after hot drawing, stretching and annealing. The crystalline
oriented form can also be obtained by high stress (high-speed) spinning.

Differential
Scanning Calorimerty (DSC) can measure crystallinity and molecular orientation
within the fibers. This type of analysis is based on distinctly different
values of the heats of fusion for crystalline and noncrystalline forms of the
polymer. The heat of fusion of the sample is compared with a calibration
standard. The crystallinity is determined by the following relationship

%
Crystallinity =

Where) is the heat of fusion
of a 100% crystalline polymer, reported in the literature to be about 33.45
cal/g (equal to 140 J/g) [4]. The Tg (glass
transition temperature) and Tm (melting point) of the fibers can
also be determined by DSC analysis. The results of the density and DSC
measurements are shown in Table 1.

The rapid
quenched PET without drawing is amorphous. The temperature range of
crystallization for PET is

From
10oC below the melting point to the temperature a little higher than
the glass transition temperature, 250-100oC. Typical PET has 50% crystallinity. The
repeat unit of PET is 1.075 nm and is slightly shorter than the length of a
fully extended chain (1.09 nm). Therefore, the chains are nearly planar. The
crystal unit cell is triclinic with dimensions a = 0.456nm, b = 0.594nm, c =
1.075nm, (= 98.5o, ß = 118o and (= 112o. [11]
PET crystal structure is illustrated in Fig. 3. Another factor for
crystallization is the position of the benzene rings. If benzene rings are
placed on the chain axis (c), then close packing of the molecular chains eases
polymer crystallization.

Fig 3.Crystal Structure of PET.

General Polyester Fiber Characteristics: [24]

·Strong

Resistant to
stretching and shrinking

Resistant to most
chemicals

Quick drying

Crisp and resilient

Wrinkle resistant

Mildew resistant

Abrasion resistant

Retains heat-set
pleats and crease

Easily washed

6. MELT-BLOWN PROCESS OF POLYESTER

The IV
(intrinsic viscosity) and crystallinity levels of a melt-blown
polyester determine the performance of the finished product. A higher IV leads
to an increased level of crystallinity, which improves the barrier properties
of the polyester melt-blown structure. However, it significantly reduces
modulus, toughness and elongation. The advantage of using polyester over such
polymers as polyolefins is its heat resistance and greater chemical resistance.
Polyesters also offer a moderate oxygen barrier.

Properties
of polyester fibers are strongly affected by fiber structure. The fiber
structure, which has a strong influence on the applicability of the fiber,
depends heavily on the process parameters of fiber formation such as spinning speed
(threadlike stress), hot drawing (stretching), stress relaxation and heat
setting (stabilization) speed.

As the
stress in the spinning threadlike is increased by higher wind-up speed, the PET
molecules are extended, resulting in better as-spun uniformity, lower
elongation and higher strength, greater orientation and high crystallinity. Hot
drawing accomplishes the same effect and allows even higher degrees of
orientation and crystallinity. Relaxation is the releasing of strains and
stresses of the extended molecules, which results in reduced shrinkage in drawn
fibers. Heat stabilization is the treatment to "set" the molecular
structure, enabling the fibers to resist further dimensional changes. Final
fiber structure depends considerably on the temperature, rate of stretching;
draw ratio (degree of stretch), relaxation ratio and heat setting condition.
The crystalline and noncrystalline orientation and the percentage of
crystallinity can be adjusted significantly in response to these process
parameters.

8. Mechanical Properties

As the
degree of fiber stretch is increased (yielding higher crystallinity and
molecular orientation), so are properties such as tensile strength and initial
Young's modulus. At the same time, ultimate extensibility, i.e., elongation is
usually reduced. An increase of molecular weight further increases the tensile
properties, modulus, and elongation. Typical physical and mechanical properties
of PET fibers are given in Table 2. And stress-strain curves in Fig. 4. It can
be seen that the filament represented by curve C has a much higher initial
modulus than the regular tenacity staple shown in curve D. On the other hand,
the latter exhibits a greater tenacity and elongation. High tenacity filament
and staple (curve A and B) have very high breaking strengths and moduli, but
relatively low elongations. Partially oriented yarn (POY) and spun filament
yarns, exhibit low strength but very high elongation (curve E). When exposing
PET fiber to repeated compression (for example, repeated bending), so-called
kink bands start to form, finally resulting in breakage of the kink band into a
crack. It has been shown in [5] that the compressibility stability of PET is
superior to that of nylons.

Table 2: Physical Properties of Polyester Fibers

Filament yarn

Staple and tow

Property

Regular tenacitya

High tenacityb

Regular tenacityc

High tenacityd

breaking tenacity,e
N/tex

0.35-0.5

0.62-0.85

0.35-0.47

0.48-0.61

breaking elongation

24-50

10-20

35-60

17-40

elastic recovery at 5%
elongation, %

88-93

90

75-85

75-85

initial modulus, N/texf

6.6-8.8

10.2-10.6

2.2-3.5

4.0-4.9

specific gravity

1.38

1.39

1.38

1.38

Moisture regian, %

0.4

0.4

0.4

0.4

Melting temperature, oC

258-263

258-263

258-263

258-263

aTextile-filament yarns for woven and knit fabrics.bTire cord and high
strength, high modulus industrial yarns.cRegular
staple for 100% polyester fabrics, carpet yarn, fiberfill, and blends with
cellulosic blends or wool.dHigh
strength, high modulus staple for industrial applications, sewing thread, and
cellulosic blends.eStandard
measurements are conducted in air at 65% rh and 22oC.fTo
convert N/text to ge/den, multiply by 11.33.gThe equilibrim moisure
content of the fibers at 21oC and 65% rh.

Shrinkage
varies with the mode of treatment. If relaxation of stress and strain in the
oriented fiber is allowed

to occur through shrinkage during fiber
manufacture, then shrinkage at the textile processing stage is reduced and
initial modulus is lowered. Polyester yarns held to a fixed length under
tension during heat treatment are less affected with change in modulus, and
reduced shrinkage values can still be obtained. This is very important in fiber
stabilization. PET shows nonlinear and time-dependent elastic behavior. It
recovers well from stretch, compression, bending, and shear
because of its relatively high initial modulus. Extensional creep occurs under
load, with subsequent delay in recovery upon removal of the load. But compared
with other melt-spun fibers, the creep is small.

The formation
of small fuzz balls of entangled fibers (pills) on the fabric surface can be a
serious problem. Fuzz formation may be affected by friction, stiffness,
breaking strength and abrasion resistance. Shape, fineness, stiffness,
recovery, friction and elongation influence entanglement of fibers. After the
pills have been formed, their rate of wear-off can affect the fabric
appearance. Wear-off is a function of fiber breaking strength and flex life. Reducing the molecular weight, which
affects the abrasion resistance; flex life, and breaking strength, results in a
decrease in pilling tendency of PET fiber. However, spinning low
molecular weight linear PET fiber is difficult. As the molecular weight is
reduced, the melt viscosity decreases and a uniform fiber with satisfactory
continuity of spinning cannot be produced. Melt viscosity can be raised by the
addition of a cross-linking compound, which is prone to hydroxyl groups.
Another property, important especially to the apparel industry, is crimp
stability or crimp compression. Generally, the tighter the packing of molecular
chains, the stiffer and more mechanically resistant the fiber is. Crimp
stability of the fiber can be improved with an increase in heating temperature.
In addition, crimp compression of the fiber can be decreased by increasing draw
ratio when the fiber is produced [6].

9. Chemical Properties

Polyester
fibers have good resistance to weak mineral acids, even at boiling temperature,
and to most strong acids at room temperature, but are dissolved with partial
decomposition by concentrated sulfuric acid. Hydrolysis is highly dependent on
temperature. Thus conventional PET fibers soaked in water at 70oC
for several weeks do not show a measurable loss in strength, but after one week
at 100oC, the strength is reduced by approximately 20%.

Polyesters
are highly sensitive to bases such as sodium hydroxide and methylamine, which
serve as catalysts in the hydrolysis reaction. Methylamine penetrates the
structure initially through noncrystalline regions, causing the degradation of
the ester linkages and, thereby, loss in physical properties. This
susceptibility to alkaline attack is sometimes used to modify the fabric
aesthetics during the finishing process. The porous structures produced on the
fiber surface by this technique contribute to higher wettability and better
wear properties [7].

Polyester
displays excellent resistance to oxidizing agents, such as conventional textile
bleaches, and is resistant to cleaning solvents and surfactants. Also, PET is
insoluble in most solvents except for some polyhalogenated acetic acids and
phenols. Concentrated solutions of benzoic acid and o-phenylphenol have a
swelling effect.

PET is both
hydrophobic and oleophilic. The hydrophobic nature imparts water repellency and
rapid drying. But because of the oleophilic property, removal of oil stains is
difficult. Under normal conditions, polyester fibers have a low moisture regain
of around 0.4%, which contributes to good electrical insulating properties even
at high temperatures. The tensile properties of the wet fiber are similar to
those of dry fiber. The low moisture content, however, can lead to static
problems that affect fabric processing and soiling.

10. Optical Properties

PET has
optical characteristics of many thermoplastics, providing bright, shiny effects
desirable for some end uses, such as silk-like apparel. Recently developed
polyester microfiber with a linear density of less than 1.0 denier per filament
(dpf), achieves the feel and luster of natural silk [23].

11. Thermal Properties

The thermal
properties of PET fibers depend on the method of manufacture. The DTA (Fig. 5.)
and TMA (Fig. 6) data for fibers spun at different speeds show peaks
corresponding to glass transition, crystallization, and melting regions. Their
contours depend on the amorphous and crystalline content. The curves shown for
600 m/min and above are characteristic of drawn fiber. The glass transition
range is usually in the range of 75oC; crystallization and melting
ranges are around 130oC and 260oC, respectively.

The thermal
degradation of PET proceeds by a molecular mechanism with random chain scission
at ester linkages, although a radical mechanism has also been proposed. A
chain-scission scheme is shown below:

The
degradation products can undergo further changes, but at ordinary processing
temperatures a certain proportion of carboxyl groups is
introduced into the polymer structure. Color formation upon degradation has
been attributed to the formation of polyenaldehydes from acetaldehyde and from
a further breakdown of poly(vinyl ester)s.

12. DYEING PROPERTIES

Because of
its rigid structure, well-developed crystallinity and lack of reactive
dyesites, PET absorbs very little dye in conventional dye systems. This is
particularly true for the highly crystalline (highly drawn), high tenacity-high
modulus fibers. Polyester fibers are therefore dyed almost exclusively with
disperse dyes.

A
considerable amount of research work has been done to improve the dyeability of
PET fibers. Polymerizing a third monomer, such as dimethyl ester, has
successfully produced a cationic dyeable polyester fiber into the
macro-molecular chain. This third monomer has introduced functional groups as
the sites to which the cationic dyes can be attached [8]. The third monomer
also contributes to disturbing the regularity of PET polymer chains, so as to
make the structure of cationic dyeable polyester less compact than that of
normal PET fibers. The disturbed structure is good for the penetration of dyes
into the fiber. The disadvantage of adding a third monomer is the decrease of
the tensile strength.

A new
dyeing process for polyester fiber at low temperature (40(C and below) has been
reported [9]. This method employs a disperse dye in a microemulsion of a small
proportion of alkyl halogen and phosphoglyceride. The main advantage of this
method is low temperature processing, but there remains the environmental
problem that is produced by using toxic carriers.

Another
approach has been introduced by Saus et al [20]. The textile industry uses
large amounts of water in dyeing processes emitting organic compounds into the
environment. Due to this problem a dying process for polyester fiber was
developed, in which supercritical CO2 is used as a transfer medium
[21]. This gives an option avoiding water discharge. It is low in cost,
non-toxic, non-flammable and recyclable. When dyed in an aqueous medium,
reduction clearing is to be carried out to stabilize color intensity, which produces
more wastewater. Reduction clearing is not carried out following supercritical
dyeing. Other advantages are better control of the dying process and better
quality of application achieved.

Spun bond
PET nonwoven webs have been treated by (SO2+O2) plasma
and (N2+H2+He) plasma at the University
of Tennessee, Knoxville. The research results show that
spun bond PET nonwovens web can be colored by conventional water-soluble acid
dyes [10]. Plasma techniques open new avenues for coloring PET fabrics and are
sure to be more evident in the coloring of polyester fibers in the future.

13. Other Properties

Polyester
fibers display good resistance to sunlight but long-term degradation appears to
be initiated by ultraviolet radiation. However, if protected from daylight by
glass, PET fiber gives excellent performance, when enhanced by an UV
stabilizer, in curtains and automobile interiors. Although PET is flammable,
the fabric usually melts and drops away instead of spreading the flame. PET
fiber will burn, however, in blends with cotton, which supports combustion.

Polyester
has good oxidative and thermal resistance. Color forming species are produced
and carboxyl end groups are increased. The resistance to both oxidative and
thermal degradation may be improved by antioxidants. Mechanical properties are
not affected by moderate doses of high-energy radiation. At doses of more than
0.5Mgy (Mrad), the tensile strength and ultimate elongation decrease, and
deteriorate rapidly at 1-5 Mgy (100-500Mrad). Finally the resistance of
polyester fibers to mildew, aging and abrasion is excellent. Molds, mildew and
fungus may grow on some of the lubricants or finishes, but do not attack the
fiber.

14. APPLICATIONS

DuPont
Company produced the first U.S.
commercial polyester fiber in 1953. Since polyester fiber has a lot of special
characteristics, most of them are used in the following three major areas: [24]

Surgeon's
gowns, for example, were once woven linen but are now for the most part
repellant treated entangled polyester fiber pulp composites on spun bond melt
blown laminates. These new gowns are far superior to the older material in
providing a breathable barrier between the surgeon and the patient, which
serves to significantly reduce hospital infections. Spun lace mattress pad
facing of 100% polyester continues to be the replacement of spun bond material
because of the textile-like character of entangled fiber fabrics. PET has
become the most important polymer type of fibrous prostheses. It is reasonably
inert, biocompatible, flexible, and resilient and has an appropriate level of
tissue acceptance. But, polymerization initiators, antioxidants, titanium
dioxide and other impurities should be minimized to improve its biocompatibility.

Thermoplastics
such as polyester are usually considered less flammable than cellulosic fibers
because they melt and shrink of drip away from the flame. Polyester resin such
as Crystar, a DuPont trade name, is used to produce spun bonded polyester in a
variety of applications: a nonwoven sheet fabric, fabric softener dryer sheets
filtration media, apparel interlining, carpet backing, furniture and bedding,
automotive seats and agricultural crop covers.

One of the
important applications of PET is in the form of bicomponent fibers. To increase
the strength of the nonwoven fabric, in while maintaining the soft hand of
LLDPE, PET is used in continuous bicomponent filaments having a sheath
component made of LLDPE and a core component made of PET. The tensile strength
of the fabrics is improved remarkably by the bicomponent filaments and depends
on the LLDPE/PET ratio. The ultrasonically bonded polyester/polypropylene blend
like Matarh's Ultraskin, the protective clothing, is said to protect wearers from
rain while offering the breathability needed to provide comfort.

Dry and wet
laid nonwovens made from a range of synthetic and inorganic fibers are used in
various insulation and industrial applications. A series of nonwoven polyester
fiber mats are used in class F (155 c) DMD flexible electrical insulation
laminates and electrical tape backing applications. Nonwoven mats made of
polyester fibers and high temperature resistant m-Aramid is used as a cost
effective replacement for Aramid paper in class H (180 C) flexible electrical
insulation composites. [28]

Composites
made of 100% polyester fibers are widely used as filtration media. Its layered
structure gives excellent tear strength, a smooth, fiber free surface and edge
stability. These products provide higher filtration efficiencies than spun
bonded media that has not been calendared. The main advantage of these products
is that they have no short fibers to be carried downstream and contaminate the
filtrate. [29]